1. Field of the Invention
The present invention relates to a corrosion resistant plated steel strip. More particularly, the present invention relates to a high corrosion and rust resistant plated steel strip having a zinc-based alloy base plating layer and thus useful for transportation vehicles, for example, cars and trucks, building materials, and electric appliance.
2. Description of Related Arts
It is known that a steel strip plated with zinc and a zinc-based alloy exhibits an enhanced resistance to corrosion and rust. This corrosion resistance of the plating layer consisting of zinc or a zinc-based alloy is mainly derived from a self-sacrificing anticorrosional action of zinc.
However, it is also known that, if a steel strip plated with zinc or a zinc-based alloy is used in a corrosional circumstance, particularly in the presence of salt, zinc is dissolved at a relatively high rate, and thus the corrosion resistance of the plated steel strip cannot be maintained at a high level.
The reasons for the above-mentioned phenomenon are as follows.
First, zinc has a higher ionization tendency and lower electric potential than those of iron. Therefore, an excessively large Zn-Fe coupling current flows, in a zinc-plated steel strip and thus zinc is dissolved at a high rate.
Second, the corrosion product of zinc has a high conductivity of the corrosion electric current, and thus the membrane of corrosion product is easily dissolved.
To avoid the above-mentioned disadvantages, attempts have been made to plate a steel strip substrate surface with a zinc-based alloy containing iron and/or nickel. The resultant plating alloy layer has a high electric potential than pure zinc and a smaller potential difference between iron and the zinc alloy than that between iron and pure zinc. This feature restricts the flow of corrosion current through the plated steel strip, and thus the plating layer can protect the steel strip substrate over a longer period.
Japanese Examined Patent Publication (Kokoku) No. 58-15,554 discloses a plated steel strip having a plating layer comprising a zinc-iron alloy or a zinc-nickel alloy. This plating layer is disadvantageous in that an iron component in the zinc-iron alloy-plating layer is corroded so as to form red rust. In the zinc-nickel alloy-plating layer, the corrosion rate of nickel is very low. This feature results in a remaining of nickel in the state of metal in the corroded plating layer, and the metallic nickel on the steel strip substrate undesirably promotes perforation corrosion of the steel strip substrate.
Japanese Unexamined Patent Publication (Kokai) Nos. 61-127,900, 61-270,398, 61-235,600 and 61-266,598 discloses a corrosion-resistant plated steel strip having a zinc-based plating layer containing alumina or silica colloidal particles dispersed therein.
However, the corrosion-preventing effect of the alumina and silica colloidal particles is unsatisfactory. Also, the alumina or silica colloidal particle-containing plating layer exhibits a poor appearance.
Japanese Examined Patent Publication No. 49-3610 and Japanese Unexamined Patent Publication No. 61-270,398 discloses a plated steel strip having a zinc-iron alloy-plating layer. This plated steel strip exhibits an enhanced corrosion resistance after being coated with an organic paint, and thus is useful for industrial purposes. However, a further enhancement of the corrosion resistance is strongly desired.
Japanese Examined Patent Publication (Kokoku) Nos. 61-36078 and 58-56039 and Japanese Unexamined Patent Publication (Kokai) No. 61-270,398 discloses a plated steel strip having a plating layer comprising co-deposited zinc and chromium, thus exhibiting an enhanced resistance to corrosion. However, the content of chromium in the plating layer is very small, and thus the corrosion resistance of the resultant plated steel strip is unsatisfactory.
In conventional co-deposition method of zinc and chromium from an electric plating liquid containing zinc ions and trivalent chromium ions, chromium can be co-deposited in a very small amount of 0.005 to 5% based on the total weight of the co-deposited zinc and chromium. An increase in the concentration of the trivalent chromium ions in the plating liquid does not increase the content of chromium in the resultant co-deposited zinc-chromium alloy plating layer, and results in a decreased adhesion of the resultant zinc-chromium alloy plating layer to the steel strip substrate and in a remarkably decreased electric current efficiency.
Accordingly, the conventional zinc-chromium alloy plating method can not be industrially utilized.
Japanese Examined Patent Publication (Kokoku) No. 58-56039 discloses that, when a zinc-chromium alloy containing 10 to 100 ppm of chromium is plated from an acid zinc plating liquid, the resultant plating layer surface has a pearl-like gloss.
Also, an increase in the content of chromium should result in an increase in the corrosion resistance of the resultant plated steel strip. However, it has been found that when the content of chromium in the zinc-chromium alloy plating layer is increased to a level of more than 1% by weight, the resultant plating layer becomes dark grey in color and exhibits uneven stripe-shaped patterns, due to the increase in the content of chromium. Therefore, the plated steel strip having a zinc-chromium alloy-plating layer containing 1% by weight of chromium is useless as a commercial product. The production of a zinc-chromium alloy plating layer having both a pearl-like gloss and an enhanced corrosion resistance is very difficult.
Further, it has been found that the increase in the content of chromium in the zinc-chromium alloy plating layer results in a decrease in the phosphate coating layer-forming property of the plating layer. That is, when a phosphate chemical conversion treatment is applied to the zinc-chromium alloy plating layer, a large content of chromium in the resultant plating layers, causes the resultant plating layer to exhibit a significantly decreased adhesion property to phosphate membrane. Accordingly, even if a painting layer is formed on the zinc-chromium alloy plating layer, the increase in the corrosion resistance of the resultant plated steel strip is unsatisfactory.
Japanese Unexamined Patent Publication (Kokai) Nos. 60-50179 and 58-98172 discloses a plated steel strip having a zinc, zinc-nickel alloy or zinc-iron alloy plating layer. The conventional plated steel strip is usually coated with an organic paint layer having a thickness of 0.5 to 2.5 .mu.m. The organic paint layer is effective for enhancing the corrosion resistance of the plated steel strip, but when the organic paint layer is cracked, the corrosion resistance of the plated steel strip is borne only by the plating layer. Therefore, the duration of the corrosion resisting activity of conventional plating layer is unsatisfactory.
Japanese Unexamined Patent Publication (Kokai) No. 61-270398 discloses an iron-zinc alloy surface plating layer formed on a zinc-based base plating layer. This iron-zinc alloy surface plating layer effectively increases the corrosion resistance of a paint-coated steel strip. However, when the iron-zinc alloy plating layer is formed on a zinc-chromium alloy base plating layer, the corrosion potential of the zinc-chromium alloy base plating layer is lower than that of the iron-zinc alloy plating layer, and thus the resultant plated steel strip sometimes exhibits an unsatisfactory corrosion resistance under a certain corrosion circumstance.
To produce a zinc-chromium alloy plating layer containing more than 5% by weight of chromium, it is important to maintain the contents of zinc ions (Zn.sup.2+) and chromium ions (Cr.sup.3+) in a plating liquid at a necessary high level.
When chromium ions (Cr.sup.3+) are fed in the form of chromium sulfate or chromium chloride into the plating liquid, the content of sulfate ions (SO.sub.4.sup.2-) or chlorine ions (Cl.sup.-) in the plating liquid is increased, and this large content of sulfate ions or chlorine ions disturbs the smoothness of the plating procedure. Chromium ions (Cr.sup.3+) cannot be fed in the form of chromium oxide (Cr.sub.2 O.sub.3) or metallic chromium, because they are not soluble in an acid plating liquid even when the liquid has a pH of 1.0 or less.
Chromium ions (Cr.sup.3+) may be fed into the plating layer in the form of chromium hydroxide (Cr(OH).sub.3) or chromium carbonate (Cr.sub.2 (CO.sub.3).sub.2), but they are only partly dissolved in the plating liquid and the non-dissolved portion thereof deposits from the plating liquid, because the hydroxide and carbonate of chromium are easily oxidized with air into chromium oxide which is insoluble in the plating liquid. Prevention of the oxidation of the chromium hydroxide and carbonate is possible but is very expensive, and thus is not industrially practical.
It is also possible to use a soluble anode consisting of metallic chromium to feed chromium ions (Cr.sup.3+) from the anode. However, in this method, metallic chromium anode is electrically dissolved in a much larger amount than a necessary amount for plating a cathode and, therefore, the content of the chromium ions (Cr.sup.3+) in the plating liquid cannot be maintained at a constant level.
Accordingly, the provision of a method effective for continuously feeding chromium ions (Cr.sup.3+) and for maintaining the content of the chromium ions (Cr.sup.3+) in the plating liquid at a required constant level is strongly desired.